In a conventional jet engine powered aircraft, an engine system is fed with intake air supplied through a forward-facing air intake. This air is then mixed with a fuel, the mixture combusted and the resultant exhaust gases are used to provide propulsion of the aircraft.
It is well known that the geometric shape of the air intake is important in ensuring efficient operation of the engine system. This is particularly important when the aircraft is travelling faster than the speed of sound in air.
Flight conditions vary considerably from take-off and subsonic flight through transonic and low Mach number supersonic flight to high Mach number supersonic flight. Consequently as flight conditions change, the geometry of the air intake is ideally varied in order to ensure that the intake air mass flow is matched to that required by the propulsion engine or engines. Such variation ensures that the air intake is running full, thereby to obtain optimum efficiency of operation as flight conditions change.
For flight at high supersonic Mach numbers the intake structure is such as to produce a plurality of shock waves ahead of an intake lip forming part of the boundary of the intake aperture, and the requirement for optimum installed engine efficiency is that the last of the shock waves (which is termed the “normal shock wave”) each of which forms a consecutive compression stage, is kept on the intake lip. If the normal shock wave enters the intake passage, a loss of efficiency occurs. Conversely, if the normal shock wave is too far ahead of the intake lip, air spillage occurs round the intake lip causing spillage drag.
Spillage drag, as the name implies, occurs when an inlet “spills” air around the outside of the intake lip instead of transferring the air to the engine's compressor. The amount of air that goes through the inlet is set by the engine and can change with altitude and throttle setting. The inlet is usually sized to pass the maximum airflow that the engine can ever demand and, for all other conditions, the inlet will spill the difference between the actual engine airflow and the maximum air demanded.
Current practice for medium supersonic Mach number intakes (M≈1.5 to 2) with external supersonic to subsonic shock diffusion systems is to operate with a final normal shock in front of the intake lip. This is achieved by over-sizing the inlet. This provides a stability margin with a low risk of aerodynamic disturbance to the power plant. This approach has a disadvantage that a percentage of the flow is spilled around the intake lip with consequent spillage drag. If the intake was sized to eliminate spillage by allowing the shock wave to sit inside the intake there is a risk of aerodynamic instability causing the shock wave to jump in and out of the intake thus disturbing the turbo machinery.
Some form of variable intake is needed if an oversized fixed intake is not employed. Variable intake schemes already devised are either more suited to higher Mach number engines, having larger, slow moving mechanical flaps and/or tend to be rectangular in section and thus more suited to fuselage mounted engines.
Alternatively, for an axi-symmetric intake having a conical centre body, the variable intake arrangement could be realised by axially displacing the centre body itself. However, for intermediate Mach numbers the cost and weight implications of such an approach relative to the benefits are prohibitive.